There are a number of applications requiring accurate frequency and/or time synchronisation references in order to properly operate. In case of frequency synchronisation a traditional solution has been to obtain synchronisation from a synchronous stream of data, as for instance in the case of Time-Division Multiplexed (TDM) based networks.
Migration of networks from TDM to packet-based technologies, such as Ethernet, requires a different approach to synchronisation. One approach to performing synchronisation across a packet based network is to send packets across the network which carry timestamp information. The timestamps are generated by a master (server) that has access to an accurate reference, such as Global Positioning System (GPS). It is possible to provide time synchronisation between a pair of nodes by using a timing protocol such as the Network Time Protocol (NTP) or Precision Time Protocol (PTP), defined in IEEE 1588. A master node with access to an accurate time source (e.g. GPS) provides a timestamp and the protocol determines the transfer delay between the master node and the slave node. One fundamental assumption with this approach is that the delay from master to slave and from slave to master is identical. This means that any asymmetry in the network would significantly impact the performance of the delivered time synchronisation reference.
An asymmetry in propagation delay can occur where forward traffic and reverse traffic (and therefore forward PTP signals and reverse PTP signals) is subject to different transmission characteristics. In an optical communications network, forward and reverse traffic may be carried by different links, having different transmission characteristics. Even in the case where forward traffic and reverse traffic are carried over the same link, the use of different wavelengths for the forward and reverse traffic can cause an asymmetry in propagation delay.
A known solution to correct for asymmetries in the links is to manually calibrate the links. If the delay asymmetry of the path connected to the ingress port is known, the corrections can be made as specified by the PTP protocol. However, this must be performed node-by-node and can be an extremely costly and time consuming process. Moreover, at any change in the network (e.g. adding transmission equipments) the compensation has to be updated. This can be a too complex and costly task creating a significant obstacle in the deployment of IEEE 1588 technology.
One proposal to distribute asymmetry information is to flood asymmetry data across a network by using Open Shortest Path First (OSPF). This can require a large amount of data and can cause scalability issues.